(1) Field of the Invention
The present invention relates to an apparatus and a method for measuring concentrations of gas components, which can measure concentrations of specific gas components even in a gas in which an oxygen partial pressure is varied.
(2) Related Art Statement
If a metal oxide semiconductor (MOS) is disposed in an atmosphere or a gas including various specific gas components, a resistance of the MOS varies. Prior methods for measuring concentrations of specific gas components have used the resistance variation mentioned above. In the method mentioned above, gas components such as CO, CO.sub.2, NO, NO.sub.2, H.sub.2, H.sub.2 O, SO.sub.2, HCl, H.sub.2 S and so on can be measured.
For example, in Japanese Patent Laid-open Publication No. 63-313047 (JP-A-63-313047), SnO.sub.2 is used as the MOS, and a concentration of H.sub.2 S is measured. In Japanese Patent Laid-open Publication No. 1-150849 (JP-A-1-150849), TiO.sub.2 is used as the MOS, and a concentration of NO is measured. In Japanese Patent Publication No. 45-117038 (JP-B-45-17038), WO.sub.3 is used as the MOS, and a concentration of NH.sub.3 is measured. In all the cases mentioned above, a concentration of the gas component is determined by detecting a resistance of the MOS in response to a variation of the specific gas concentration, and comparing the resistance thus detected with a predetermined reference curve showing a relation between a resistance and a gas concentration of the MOS.
Generally, the oxygen concentration of an exhaust gas discharged from an internal combustion engine or an exhaust gas discharged from a dust incinerator varies greatly. In these exhaust gases, we find that the resistance of the MOS varies greatly in response to a variation of an oxygen partial pressure in the exhaust gas as well as the specific gas concentration. Moreover, the detected resistance is influenced by both the specific gas concentration and the oxygen partial pressure. Therefore, it is not possible to detect a concentration of the specific gas component precisely. For example, as shown in FIG. 4 mentioned below, if the oxygen partial pressure is varied from 0.01 atm to 0.20 arm, resistivities of the MOS therebetween are largely different from each other while concentration of NO therebetween are identical with each other.